Modern communications demand high density, multifunction monolithic microwave integrated circuits (MMICs). MMiCs use microstrip and coplanar waveguide as the planar connections between semiconductor devices. These open waveguides contain a significant portion of electromagnetic energy above the circuit substrate. As advanced circuits are made more complex and more compact, interelement coupling begins toaffect overall circuit performance. For example, if a microstrip were placed too close to a microstrip spiral inductor that is used in a filter, the parasitic coupling between the two elements could change the value of the inductor and detune the filter. Although MMIC density is still low relative to that of most advanced digital integrated circuits, MMIC designers are faced with competing objectives. A circuit design must simultaneously accommodate the area required by the largest structures such as inductors or inductive lines in an MMIC, maintain a reasonable distance from nearby transmission lines and other elements, and minimize the use of chip real estate.
A grounded transmission line has been shown to substantially decouple closely spaced transmission lines reducing the level of crosstalk by as much as 10 dB as disclosed by iM. Le Brun, P. R. Jay, and C. Rumelhard, Strip Cuts Coupling on Crowded MMiC Chips, Microwaves, 79-80, October 1981. This article does not discuss active device performance, but only transmission line performance, and therefore does not teach or address transistor source electrode configurations.
The MESFET can be modeled to predict performance characteristics. The MESFET has been modeled as a distributed device and an analytical technique has used to model traveling waves.
Calculated results, using resistive terminations, produced gain curves. Using resistive terminations, gain curves can be produced as found by W. Heinrich and H. Hartnagel, Wave, Propagation on MESFET Electrodes and Its Influence on Transistor Gain, IEEE Trans. Microwave Theory Tech., Vol. MTT-35, No. 1, 1-8, January 1987. This article does not discuss ways of configuring a transistor to enhance its shielding properties and therefore does not teach or address transistor source topologies.
A [Z] matrix for two transmission lines has been calculated. Modeling current and voltage eigenvectors have been combined to produce impedance matrix [Z.sub.imp ], where V.sub.p =[Z.sub.imp ] Ip. V.sub.p and I.sub.p are voltages and currents at ports of a MESFET gain section. W. Heinrich, Distributed Equivalent-Circuit Model for Traveling-Wave FET Design, IEEE Trans. Microwave Theory Tech., Vol. MTT-35, No. 5,487-491, May 1987. This work describes MESFET models that were developed by an electromagnetic wave propagation analysis. This article does not discuss ways of configuring a transistor to enhance its shielding properties and therefore does not teach or address transistor source topologies.
A "T" cross-section configuration for a gate electrode results in reduced resistance and provides greater operating bandwidth. Growing waves have been calculated. N. Sebati, P. Gamand, C. Varin, F. Pasqualini, and J. C. Meunier, Continuous Active T-Gate Travelling-Wave Transistor, Electronics Letters, Vol. 25, No. 6 , March 1989. This work uses the traveling wave analysis for high frequency MESFETs. This article does not discuss ways of configuring a transistor to enhance its shielding properties and therefore does not teach or address transistor source topologies.
"T" gate structures have been disclosed elsewhere. S. D. D'Agostino, G. D'Inzeo, and L. Tudini, Analytical Modeling and Design Criteria for Traveling-Wave FET Amplifiers, IEEE Trans. Microwave Theory Tech., Vol. MTT-40, No. 2, 202-208, February 1992. This work discloses a drain to source separation of 140 .mu.m and a drain width of 300 .mu.m, and uses another description of the theoretical model and design of a traveling wave MESFET. This work does not discuss ways of configuring a transistor to enhance its shielding properties and therefore does not teach or address transistor source topologies.
"T" gate dimensions have been used to evaluate microwave coupling of conductors on GaAs substrates. M. Le Brun, P. R. Jay, and C. Rumelhard, Coupling and Impedance Between Line and Ground Electrodes on GaAs: Implications for MMIC Design, Proc. 11th European Microwave Conference, Amsterdam, 850-855, September 1981. This article does not discuss active device performance and therefore does not teach or address transistor source configuration.
"T" gate dimensions have been used elsewhere. G. D'Inzeo, R. Guisto, and C. Petrachi, Active Devices for Microwave Distributed Amplification, Microwave and Optical Technology Letters, Vol. 3, No. 2, 51-54, February 1990. This work discusses a drain to source separation of 210 .mu.m and a drain width of 500 .mu.m which were required to produce growing waves which may not be suitable for producing a compact transistor for MMIC applications.
A dual gate FET as been described as a major advantage over single gate FET. Such devices can be used as microwave mixers as described in S. Maas, Microwave Mixers, Artech House, Norwood, Mass., 1986. This work does not discuss way of configuring a mixer FET to enhance its shielding properties and therefore does not teach or address transistor source topologies.
Coplanar waveguides on semi-insulating GaAs substrates have been disclosed with reduced size and low wave attenuation. Seki and H. Hasegawa, Cross-tie slow-wave coplanar waveguide on semi-insulating GaAs substrates, Electon. Lett., vol. 17, no. 25, pp. 940-941, December 1981. This work does not discuss active device performance and therefore does not teach or address transistor source configurations.
Coplanar structures situated atop semiconductor substrates have been described and have wide application to coplanar monolithic microwave integrated circuits. Y. Fukuoka, and T. Itoh, Slow-wave coplanar waveguide on periodically doped semiconductor substrate, IEEE Trans. Microwave Theory Tech., vol. MTT-31, pp. 1013-1017, December 1983. Y. Fukuoka and T. Itoh, Design consideration of uniform and periodic coplanar Schottky variable phase shifter, Proc. 13th Eur. Microwave Conf., pp. 278-282, September 1983. These references do not discuss active device performance and therefore do not teach or address transistor source configurations.
MESFET device modeling has been used in the design process and verification of MMICs. Compact MMICs may disadvantageously suffer from electromagnetic coupling which occurs between various components such as planar waveguides and transistor electrodes. In conventional small signal MESFET layouts, source and drain electrodes are opposite each other, so that the drain electrode is particularly susceptible to undesired coupling. Even modest amounts of parasitic coupling between a nearby microstrip line and a small signal MESFET amplifier may lead to oscillations, degrade circuit balance or distort gain flatness. Workshop J, GaAs MMIC System Insertion and Multifunction Chip Design Issues and Trends, 1991 IEEE MTT-S International Microwave Symposium, Boston, Mass., Jun. 14, 1991. This reference does not teach or discuss the use of a transistor source electrode as a form of on chip shielding. These and other disadvantages of electromagnetic coupling problems are solved or reduced using the present invention.